1. Field of the Invention
This invention relates to an oxygen detector for detecting concentration of oxygen in exhaust gas delivered from various kinds of combustion apparatus inclusive of an internal combustion engine or the like.
2. Description of the Prior Art
An oxygen detector for detecting concentration of oxygen in exhaust gas delivered from various kinds of combustion apparatus inclusive of an internal combustion engine or the like and including an oxygen detection element composed of a solid electrolyte has heretofore been widely used.
In such kind of oxygen detector, provision is made of a tubular oxygen detection element formed of zirconia or the like and closed at one end thereof. The oxygen detection element is provided at its outer and inner peripheral walls with outer and inner electrode layers, respectively, each extending upwardly from the closed end of the oxygen detection element and formed of platinum deposited thereon by printing or the like. The oxygen detection element must be sealed and electrically conductive with a metal casing which is provided with small holes through which flows exhaust gas which makes contact with the outer electrode layer. Meanwhile, outside air must be introduced through the inner electrode into the closed end of the oxygen detection element and at the same time a lead wire connected to the inner electrode layer must be led to the outside. In this case, the outer electrode layer can easily be sealed with the metal casing, but means for allowing the outside air to introduce into the inner electrode layer of the oxygen detection element and at the same time allowing the lead wire connected to the inner electrode layer to lead out upwardly involves various drawbacks.
In order to eliminate such drawbacks, an attempt has been made that the inner electrode layer is provided at its open end with a threaded portion with which a coil-shaped terminal having an outer diameter which is slightly larger than the inner diameter of the tapped portion is threadedly engaged under pressure and that a lead wire extending upwardly from the coil-shaped terminal is connected to one of output delivery conductors. Such attempt is satisfactory under a normal condition, but has the drawback that there is a risk of the electrical conduction between the oxygen detection element and the inner electrode layer being deteriorated under a high temperature condition.
That is, the above mentioned coil-shaped terminal is preferably formed of a spring material consisting of stainless steel such as SUS631J1 having an excellent heat resistant property. The inner diameter of the open end and the diameter of the thread provided therein of the oxygen detection element are limited for the sake of construction and the coil-shaped terminal threadedly engaged under pressure with the threaded open end of the oxygen detection element must be formed of a wire having a small diameter for the sake of the threaded engagement. Under such circumstances, if the oxygen detector is used at a high temperature for a long time, the coil-shaped terminal becomes softened to reduce its tensile force when it is threadedly engaged with the threaded open end of the oxygen detection element, thereby deteriorating the electrical conduction between the oxygen detection element and the inner electrode layer. For example, if a coil-shaped terminal formed of a spring material consisting of SUS631J1 and having a diameter of 0.6 mm was used at 300.degree. C., its tensile force was not reduced, but if it was continuously used at 400.degree. C. for 100 hours, its tensile force was reduced.